A method for preparing a flexible oxide memristor array using an electrochemical process

Abstract

The present invention relates to a method for preparing a flexible oxide memristor array using electrochemical anodization technology, which belongs to the intersection of flexible semiconductor devices and electrochemistry. A plurality of mutually independent inert bottom electrodes are deposited on a flexible substrate using a mask; active metal layers are deposited on the obtained inert bottom electrodes using a mask; connecting rods are added to each inert bottom electrode to connect all the inert bottom electrodes into a whole to ensure uniformity and integrity during current conduction; the connecting rods are connected to the positive pole of the power supply, and all the active metal layers are completely immersed in an electrolyte for anodization, so that the active metal layers are oxidized into an oxide intermediate layer; a top electrode is deposited on the oxide intermediate layer, and then the connecting rods are removed to obtain a flexible oxide memristor array. The present invention adopts a fast and convenient process to prepare a flexible memristor array, which improves the efficiency and uniformity of preparing the memristor array by anodization.

Description

Technical Field

[0001] The present invention relates to the interdisciplinary field of flexible micro-nano devices and electrochemistry, and more specifically, to a method for preparing a flexible oxide memristor array using an electrochemical process. Background Art

[0002] With the rapid development of big data, the Internet of Things, and edge devices, the amount of information being transmitted and transmitted is exploding, placing higher demands on the ability to quickly transmit and process information. Current computer architectures still rely on the von Neumann architecture, where the central processing unit (CPU) processes data and transmits it to memory via a data bus. This mismatch between the processor's processing speed and the memory's speed leads to wasted computing power and delays in data transmission. A new generation of integrated storage and computing architectures aims to address these issues by integrating storage and computing into a single unit. Memristors, a fourth component, are electronic devices with adjustable conductance that can be easily adjusted and stored. Resistive random access memory (ReRAM) fabricated from memristors has been initially used in integrated storage and computing chips, significantly reducing chip power consumption. Memristors also possess a volatile property that mimics the function of neurons in the brain.

[0003] In flexible electronic applications, flexible ReRAM devices offer numerous advantages, such as foldability and lightweightness. They can be used in flexible displays and wearable devices, offering several distinct advantages over non-flexible ReRAM. Traditionally, flexible memristors have been fabricated using deposition techniques such as physical vapor deposition (PVD), chemical vapor deposition (CVD), atomic layer deposition (ALD), and pulsed laser deposition (PLD). While these methods offer a certain degree of uniformity and good precision, they are generally expensive (over hundreds of thousands of yuan), require demanding conditions, require long cycles, and sometimes require high temperatures. Anodization is an electrochemical process in which a metal anode in an electrolyte loses electrons under the influence of an electric current, oxidizing it to an oxide. This technique is simple and convenient to implement, with inexpensive equipment (less than 1,000 yuan), making it within the reach of most researchers. Furthermore, samples produced using this technique exhibit a gradient distribution of oxygen vacancies and a uniformly oxidized surface, enabling the production of stable flexible memristors.

[0004] In the traditional anodization process, the metal to be oxidized is immersed in a solution, while the remaining homogeneous metal is kept in the solution and connected to the positive electrode of the power supply to facilitate current conduction. This prevents oxidation of the connection between the positive electrode and the metal, which would cause a sharp increase in resistance and thus prevent current conduction. However, in the process of fabricating memristors using anodization, the electrolyte can easily splash onto the positive electrode connection due to the need for solution stirring and the miniaturization of the device, causing oxidation and poor contact.

[0005] When fabricating flexible memristor arrays, when the bottom electrodes are arranged in multiple, separate configurations, it's easy for the bottom electrodes to not fully contact the positive power source (incomplete contact between the positive electrode clip and the bottom electrode), preventing current from reaching all the intermediate layers. Excessively dispersed electrode distribution can lead to multiple connections between the positive power source and the bottom electrode, oxidizing all the intermediate layers and reducing the oxidation success rate and efficiency. Summary of the Invention

[0006] In response to the above-mentioned deficiencies or improvements in the prior art, the present invention provides a method for preparing flexible oxide memristors and arrays thereof using an electrochemical process. The purpose is to combine electrochemical processes to prepare flexible memristors, improve memristor preparation efficiency, and reduce their preparation cost and complexity. First, because photolithography typically requires heating and drying the photoresist, which can cause deformation of the flexible substrate, this technology uses a metal mask to prepare a flexible memristor array. To address the problem of electrolyte easily sputtering onto the sample bottom electrode and the positive power supply during oxidation of small-sized devices, the present invention uses an inert electrode as the bottom electrode and connects the bottom electrode to the positive power supply. This ensures that even if the bottom electrode is completely immersed in electrolyte, it will not oxidize, ensuring good current conduction. Furthermore, to address the problems of poor contact caused by the dispersed distribution of the bottom electrode and uneven oxidation of all intermediate layers, a connecting rod is used to connect all arrays of bottom electrodes into a single entity, ensuring uniform oxidation current across all intermediate layers. The present invention produces flexible memristors and arrays with stable performance, which can be applied to fields such as neuromorphic simulation, flexible wearable electronic devices, and flexible electronic screens.

[0007] According to a first aspect of the present invention, a method for preparing a flexible oxide memristor array by an electrochemical process is provided, comprising the following steps:

[0008] (1) Using a mask to deposit multiple independent inert bottom electrodes on a flexible substrate;

[0009] (2) using a mask to deposit active metal layers on the inert bottom electrodes obtained in step (1); the material of the active metal layers is a metal that can be oxidized among transition metals;

[0010] (3) adding a connecting rod to each inert bottom electrode to connect all the inert bottom electrodes into a whole to ensure uniformity and integrity during current conduction; connecting the connecting rod to the positive electrode of the power supply, and completely immersing all the active metal layers in the electrolyte; using the active metal layer as the anode and platinum, gold, graphite or stainless steel as the cathode, performing anodic oxidation, and conducting current through the inert bottom electrode to the active metal layer, so that the active metal layer is oxidized into an oxide intermediate layer;

[0011] (4) depositing a top electrode on the oxide intermediate layer and then removing the connecting rods to obtain a flexible oxide memristor array.

[0012] Preferably, the flexible substrate is a polyethylene naphthalate film, a polyethylene terephthalate film, a polyimide film, a polymethyl methacrylate film or a cellulose film.

[0013] Preferably, the material of the inert bottom electrode is Pt, Au or Pd, so that the flexible substrate is completely immersed in the electrolyte and the part connected to the positive electrode of the power supply is not oxidized, thereby continuously transmitting current to the active metal layer.

[0014] Preferably, the material of the active metal layer is Ta, Hf, Zn, Al, Mg, Ti, Cu, Ni, Nb or Zr.

[0015] Preferably, the active metal layer has a thickness of 10-500 nm.

[0016] Preferably, the voltage range of the anodic oxidation is 5-24V.

[0017] Preferably, the anodizing time is 5 seconds to 5 minutes.

[0018] Preferably, the cathode is a sheet or mesh structure, and has an area larger than that of the flexible substrate.

[0019] According to another aspect of the present invention, a flexible oxide memristor array prepared by any one of the methods is provided.

[0020] According to another aspect of the present invention, there is provided an application of the flexible oxide memristor array in neuromorphic computing, brain function simulation or non-volatile storage.

[0021] In general, the above technical solutions conceived by the present invention have the following technical advantages compared with the existing technology:

[0022] (1) The present invention allows the entire flexible substrate to be immersed in an electrolyte by clamping an inert bottom electrode, making oxidation more convenient; at the same time, the inert electrode will not be oxidized, allowing current to be continuously conducted to the middle layer, thereby preparing a cross-array memristor with different oxidation degrees. The mask shape design in the present invention can ensure that all bottom electrodes are connected into a whole through the connecting rod, so that the current is evenly conducted to the middle layer; it avoids uneven oxidation caused by poor contact between some bottom electrodes and the power anode, thereby increasing oxidation uniformity and oxidation efficiency.

[0023] (2) The flexible memristor prepared by the present invention has good stability and uniformity, and can be used to simulate brain synaptic functions, neuron functions, or realize non-volatile storage and other functions.

[0024] (3) The present invention combines an electrochemical anodization process to prepare a flexible memristor, which has a fast preparation rate and only takes a few seconds to a few minutes to prepare the functional layer required for the memristor. In addition, the preparation cost is greatly reduced compared to traditional processes.

[0025] (4) The present invention has universal applicability. Using the method of the present invention, various types of flexible memristors, such as conductive volatile type and non-volatile type, can be prepared. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 The present invention relates to a method for preparing a flexible oxide memristor and its array by an electrochemical process.

[0027] Figure 2 This is a top view of the structure of 2*2 and 4*4 cross arrays of flexible memristors prepared by electrochemical oxidation provided by the present invention.

[0028] Figure 3 This is a top view of the structure of a flexible memristor cross array prepared by electrochemical oxidation provided by the present invention.

[0029] Figure 4 This is a physical picture of the flexible memristor prepared in Example 1 of the present invention.

[0030] Figure 5 This is an IV curve diagram of the memristor device after bending according to Example 1 of the present invention.

[0031] Figure 6 This is a 100-cycle IV cycle curve of the volatile memristor provided in Example 2 of the present invention.

[0032] Figure 7 This is the conductivity retention characteristic of the volatile memristor under a read voltage provided by Example 2 of the present invention.

[0033] Figure 8 This is the current response of the volatile memristor provided in Example 2 of the present invention under a pulse voltage.

[0034] Figure 9 This is an IV cycle curve diagram of the non-volatile memristor provided in Example 3 of the present invention.

[0035] Figure 10 The conductance of the non-volatile memristor provided in Example 3 of the present invention is in high and low resistance states under a pulse voltage.

[0036] Figure 11 This is a 128-cycle IV cycle curve of the volatile memristor provided in Example 4 of the present invention. DETAILED DESCRIPTION

[0037] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely for the purpose of explaining the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0038] A method for preparing a flexible oxide memristor array by an electrochemical process comprises the following steps:

[0039] (1) Using a mask to deposit multiple independent inert bottom electrodes on a flexible substrate;

[0040] (2) using a mask to deposit active metal layers on the inert bottom electrodes obtained in step (1); the material of the active metal layers is a metal that can be oxidized among transition metals;

[0041] (3) adding a connecting rod to each inert bottom electrode to connect all the inert bottom electrodes into a whole to ensure uniformity and integrity during current conduction; connecting the connecting rod to the positive electrode of the power supply, and completely immersing all the active metal layers in the electrolyte; using the active metal layer as the anode and platinum, gold, graphite or stainless steel as the cathode, performing anodic oxidation, and conducting current through the inert bottom electrode to the active metal layer, so that the active metal layer is oxidized into an oxide intermediate layer;

[0042] (4) depositing a top electrode on the oxide intermediate layer and then removing the connecting rods to obtain a flexible oxide memristor array.

[0043] In some embodiments, the flexible substrate material is various non-conductive organic, inorganic or composite flexible films such as polyethylene naphthalate (PEN) film, polyethylene terephthalate (PET) film, polyimide thin film (PDMS) film, polymethyl methacrylate (PMMA), cellulose film or flexible films of other materials.

[0044] In some embodiments, the bottom electrode in step (1) needs to be an inert metal, such as Pt, Au, Pd and other non-oxidizable metals, to ensure that the part of the flexible substrate connected to the positive electrode of the power supply is not oxidized when it is completely immersed in the electrolyte, and the current is continuously transferred to the active metal layer.

[0045] In some embodiments, the active metal layer in step (2) is made of Ta, Hf, Zn, Al, Mg, Ti, Cu, Ni, Nb, Zr, etc. The active metal layer is to cover the bottom electrode, which is generally square or circular.

[0046] In some embodiments, the active metal layer in step (2) has a thickness of 10-500 nm, and the metal layer is a composite of one or more layers.

[0047] In some embodiments, the electrolyte in step (3) is an acid solution, an alkaline solution, a salt solution, or an alcohol solution. The concentration of the solute in the electrolyte is 0.05M / L to 1M / L. The temperature of the electrolyte is 10 to 20°C. Too high a temperature will cause excessive heat generation on the membrane surface and damage the surface morphology. Moreover, the electrolyte cannot react with the substrate material, thereby affecting the oxidation process. For example, a relatively concentrated strong oxidizing acid (concentrated sulfuric acid, concentrated hydrochloric acid, concentrated nitric acid, etc.)

[0048] In some embodiments, in step (3), the oxidation voltage generally must meet two conditions: first, the oxidation voltage cannot exceed the limit value of film shedding. If the voltage is too high, the surface current will be too large to ablate the surface of the film, causing film shedding or pores; second, the voltage cannot be too low. If the voltage is too low, the degree of metal oxidation will be too low, causing ohmic contact between the electrodes and thus failing to produce the resistive switching effect of the memristor. The voltage range is 5-24V; the oxidation time is generally between 5s and 5min. While ensuring that the flexible film will not be overoxidized and produce bubbles, flexible devices can also be quickly prepared.

[0049] In some embodiments, in step (4), the top electrode material can be an active metal, an inert metal, an oxide, a nitride, etc., and the thickness is 10 to 200 nm; the top electrode position should cover the middle layer and should not contact the bottom electrode to avoid direct short circuit.

[0050] In some embodiments, the active metal layer is one or more of copper, tantalum, titanium, nickel, tungsten, hafnium, niobium, yttrium, zinc, cobalt, aluminum, silicon, and zirconium.

[0051] The flexible memristor prepared according to the method of the present invention and its application in neuromorphic computing, brain function simulation or non-volatile storage.

[0052] Figure 1 The present invention relates to a method for preparing a flexible oxide memristor and its array by an electrochemical process.

[0053] When preparing a flexible memristor array, when the bottom electrode is a plurality of separately arranged shapes, such as Figure 2 and Figure 3 When the electrodes are too dispersed, it is easy to cause the bottom electrode and the positive electrode of the power supply to not make good contact (the positive electrode clamp and the bottom electrode are not in full contact), and thus the current cannot be conducted to all the intermediate layers. The electrode distribution is too dispersed, which will cause the positive electrode and the bottom electrode to connect multiple times to oxidize all the intermediate layers, reducing the success rate and efficiency of oxidation.

[0054] The shapes of the bottom and top electrodes form a cross array, and the overlapping area is a square, e.g. Figure 2 As shown. Figure 2 and Figure 3 As mentioned above, the independent bottom electrodes must be connected as a whole via connecting rods to ensure uniform current conduction when the intermediate layer is oxidized.

[0055] The following are specific embodiments

[0056] Example 1

[0057] This embodiment provides a method for preparing a flexible oxide memristor and an array thereof by an electrochemical process, comprising the following steps:

[0058] (1) Preparation of bottom electrode: Place the cleaned flexible PDMS substrate in a magnetron sputtering device. The target materials are Ti and Pt, with Pt as the bottom electrode. Pump the vacuum of the chamber to 2*10 -6 At a pressure below 1 Torr, high-purity argon was introduced to sputter 10 nm Ti and 100 nm Pt. The sputtering power for Ti was 25 W, and for Pt was 20 W.

[0059] (2) Preparation of composite metal active layer: Place a square mask on the flexible substrate in step (1), install the Nb target, introduce high-purity argon gas, and sputter 18nm of metal Nb at a power of 20W.

[0060] (3) Preparation of anodizing electrolyte: Prepare 0.5 M / L dilute phosphoric acid, and then stir it with a stirring device for 2 hours to mix it evenly.

[0061] (4) Oxidation of the Metal Layer: Clamp the bottom electrode of step (1) with the electrode clamp of the positive electrode of the power supply, immerse the entire flexible substrate in the electrolyte, and then oxidize the Nb metal deposited in step (2) at a constant voltage of 23.5 V for 1 minute. Then rinse with deionized water and blow dry for later use.

[0062] (5) Top electrode preparation: A circular metal mask is placed on a flexible substrate, and then 100nm of Pt and 100nm of Au top electrodes are deposited in a magnetron device. Finally, a PDMS / Ti / Pt / NbOx / Pt / Au flexible memristor is prepared. Figure 4 This is a physical picture of the flexible memristor prepared in this embodiment.

[0063] Figure 5This is the IV curve of the device after bending. When voltage is applied to the bottom electrode, oxygen vacancy filaments form inside the device, resulting in a low resistance and the onset of the Set process. Conversely, when voltage is applied to the top electrode, the combined effects of internal heat accumulation and voltage field strength cause the oxygen vacancy filaments to break, and the device becomes high-resistance. This shows that the flexible device can maintain the resistive switching performance of the memristor after bending.

[0064] Example 2

[0065] This embodiment provides a method for preparing a flexible oxide memristor and an array thereof by an electrochemical process, comprising the following steps:

[0066] (1) Preparation of bottom electrode: Place the cleaned flexible PET substrate in a magnetron sputtering device. The target materials are Ti and Pt, with Pt as the bottom electrode. Pump the vacuum of the chamber to 2*10 -6 At a pressure below 1 Torr, high-purity argon was introduced to sputter 10 nm Ti and 100 nm Pt. The sputtering power for Ti was 35 W and that for Pt was 20 W.

[0067] (2) Preparation of a composite metal active layer: A square mask was placed on the flexible substrate prepared in step (1), and Ta and Ti targets were installed. High-purity argon gas was introduced, and 60 nm of Ta metal was sputtered at a power of 30 W. Then, 1 nm of Ti was sputtered at a power of 20 W to prepare a composite metal layer.

[0068] (3) Same as step (3) of Example 1.

[0069] (4) Oxidation of the metal layer: Clamp the bottom electrode of step (1) with an electrode clamp at the positive pole of a power supply, immerse the entire flexible substrate in an electrolyte, and then oxidize the composite metal layer deposited in step (2) at a constant voltage of 7 V for 20 seconds. Rinse with deionized water and blow dry for later use.

[0070] (5) Same as step (5) of Example 1.

[0071] The 4200 semiconductor characteristics analysis system is used to test its electrical performance. Figure 6 The IV curve under cyclic scanning shows that the device has good stability under 100 cycles and no large variability occurs under 100 cycles. Figure 7 As shown in the figure, after testing the device's conductivity retention performance, it was found that after a reverse scan, the device resistance became low, and then the device resistance would spontaneously return to high resistance after 200 seconds. It can be seen that the performance of this device belongs to a volatile memristor, which can be used for storage pool calculation neural networks, neuron function simulation, etc. Figure 8 As shown, when pulse voltage stimulation is performed, it can be seen that its conductance can be adjusted and the device has multiple conductance states.

[0072] Example 3

[0073] (1) Preparation of bottom electrode: Place the cleaned flexible PET substrate on the upper surface of the substrate with Figure 2 The metal mask with the bottom electrode shape shown is then placed in the magnetron sputtering equipment. The target materials are Ti and Pt, where Pt is used as the bottom electrode. The vacuum degree of the chamber is pumped to 2*10 -6 At a pressure below 1 Torr, high-purity argon was introduced to sputter 10 nm Ti and 100 nm Pt. The sputtering power for Ti was 35 W and that for Pt was 20 W.

[0074] (2) Preparation of metal active layer: Place a metal active layer on the flexible substrate in step (1) Figure 2 The square mask shown is installed with a Ta target, high-purity argon is introduced, and a 60nm metal Ta layer is sputtered at a power of 30W.

[0075] (3) Preparation of anodizing electrolyte: Prepare 0.4 M / L dilute phosphoric acid and 0.1 M / L ethylene glycol, and then stir with a stirring device for 2 h to mix them evenly.

[0076] (4) Oxidation of the Metal Layer: Clamp the bottom electrode of step (1) with an electrode clamp at the positive pole of a power supply, immerse the entire flexible substrate in an electrolyte, and then oxidize the composite metal layer deposited in step (2) at a constant voltage of 15 V for 15 seconds. Rinse with deionized water and blow dry for later use.

[0077] (5) Top electrode preparation: Place the device prepared in step (4) on top of the top electrode. Figure 2 The top electrode shape is shown, and then 100nm Pt and 100nm Au top electrodes are deposited in the magnetron device.

[0078] (6) The connecting rod of the bottom electrode is removed to prepare the final cross-array flexible memristor.

[0079] Figure 9 This is the IV cycle curve of the flexible memristor. It can be seen that the device has good stability and the Set voltage is within 1V. Figure 10 Figure 3 is the conductivity change of the device under positive and negative pulse voltages (+1V and -2.5V). It can be seen that the high and low resistance states of the device are still stable after 200 cycles.

[0080] Example 4

[0081] (1) Same as step (1) of Example 2;

[0082] (2) Same as step (2) of Example 2;

[0083] (3) Same as step (3) of Example 2;

[0084] (4) Oxidation of the metal layer: Clamp the bottom electrode of step (1) with the electrode clamp of the positive electrode of the power supply, immerse the entire flexible substrate in the electrolyte, and then oxidize the composite metal layer deposited in step (2) under the oxidation condition of 7V constant voltage for 30s. Then rinse with deionized water and blow dry for use;

[0085] (5) Same as step (5) of Example 2.

[0086] The performance of the device prepared using the process of this embodiment is similar to that of Example 2. Figure 11 The IV cycle curve of 128 cycles for this example shows very good cycle consistency.

[0087] Comparative Example 1

[0088] The difference between Comparative Example 1 and Example 2 is that the oxidation voltage in step (4) is 25 V. The surface of the film obtained by oxidation is characterized by multiple holes and detached film. This is because the high oxidation voltage causes local overheating, which, combined with the surface bubbles, forms holes. Therefore, the memristor under this process cannot be used normally.

[0089] It will be easily understood by those skilled in the art that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing a flexible oxide memristor array by an electrochemical process, characterized in that: The following steps are involved: (1) Using a mask to deposit multiple independent inert bottom electrodes on a flexible substrate; (2) using a mask to deposit active metal layers on the inert bottom electrodes obtained in step (1); the material of the active metal layers is a metal that can be oxidized among transition metals; (3) adding connecting rods to each inert bottom electrode to connect all the inert bottom electrodes into a whole to ensure uniformity and integrity during current conduction; connecting the connecting rods to the positive electrode of the power supply, and completely immersing all the active metal layers in the electrolyte; using the active metal layer as the anode and platinum, gold, graphite or stainless steel as the cathode, anodizing is performed, and the current is conducted to the active metal layer through the inert bottom electrode, so that the active metal layer is oxidized into an oxide intermediate layer; the voltage range of the anodizing is 5-24 V; (4) Depositing a top electrode on the oxide intermediate layer and then removing the connecting rods to obtain a flexible oxide memristor array.

2. The method for preparing a flexible oxide memristor array by an electrochemical process according to claim 1, wherein: The flexible substrate is a polyethylene naphthalate film, a polyethylene terephthalate film, a polyimide film, a polymethyl methacrylate film or a cellulose film.

3. The method for preparing a flexible oxide memristor array by electrochemical process according to claim 1, characterized in that: The material of the inert bottom electrode is Pt, Au or Pd, so that the flexible substrate is completely immersed in the electrolyte and the part connected to the positive electrode of the power supply is not oxidized, thereby continuously transmitting current to the active metal layer.

4. The method for preparing a flexible oxide memristor array by electrochemical process according to claim 1, characterized in that: The material of the active metal layer is Ta, Hf, Zn, Al, Mg, Ti, Cu, Ni, Nb or Zr.

5. The method for preparing a flexible oxide memristor array by electrochemical process according to claim 1, characterized in that: The thickness of the active metal layer is 10-500 nm.

6. The method for preparing a flexible oxide memristor array by electrochemical process according to claim 1, characterized in that: The anodizing time is 5 s to 5 min.

7. The method for preparing a flexible oxide memristor array by electrochemical process according to claim 1, characterized in that: The cathode is in a sheet or mesh structure and has an area larger than that of the flexible substrate.

8. A flexible oxide memristor array prepared by the method according to any one of claims 1 to 7.

9. Application of the flexible oxide memristor array according to claim 8 in neuromorphic computing, brain function simulation or non-volatile storage.

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